In the field of engines used in vehicles, for instance, a widely-known exhaust turbocharger rotates a turbine by energy of exhaust gas of an engine, then compresses intake air by a centrifugal compressor directly connected to the turbine via a rotational shaft, and supplies the compressed air to the engine in order to improve the output of the engine.
As represented by the normal compressor of the performance-characteristic comparison chart in FIG. 11 where y-axis is the pressure ratio and x-axis is the flow rate, a compressor of such an exhaust turbocharger is stably operated in the flow-rate range from a surge flow rate (left-hand line in the drawing) at which surging, or pulsation of the entire system, occurs, to a choke flow rate (right-hand line in the drawing) at which choking occurs and the flow rate stops increasing.
However, in a centrifugal compressor of the normal compressor type in which intake air is directly introduced into an impeller wheel, the flow-rate range between the choke flow rate and the surge flow rate where stable operation is possible is narrow. Thus, there is a problem in that it is necessary to operate the compressor at an inefficient operation point which is differed from the surge flow rate, in order to avoid surging.
In order to solve the above problem, Patent Document 1 discloses a technique of increasing the operation range of an exhaust turbocharger by providing guide vanes at the upstream side of an impeller wheel of the centrifugal compressor to swirl intake air at the upstream side of the impeller wheel, and a technique of providing a recirculation flow path for a housing of a supercharger to recirculate a part of intake gas introduced into the impeller wheel.
Such techniques will be described briefly in reference to FIG. 10.
An impeller wheel 101 of a centrifugal compressor 100 includes a plurality of vanes 104 which are rotatable in a housing 102, and the housing 102 includes an inner wall disposed in the vicinity of radially outer edges 104a of the vanes 104.
An intake-gas inlet of the centrifugal compressor 100 includes an outer annular wall 107 forming a gas inlet 108, and an inner annular wall 109 extending inside the outer annular wall 107 to form an inducer part 110. An annular gas flow channel 111 is formed between the annular walls 109, 107.
A housing surface 105 by which the vanes 104 pass through is in communication with the annular flow channel 111 via a downstream opening part 113.
An upstream opening part brings the annular flow channel 111 into communication with the inducer part 110 being the inlet intake part. Inlet guide vanes 114 are provided inside the inducer part 110 downstream with respect to the upstream opening part to induce precedent swirls in the gas flow passing through the inducer part 110. When the flow rate of the air passing through the compressor is small due to the above configuration, the direction of the air flow passing through the annular flow channel 111 is reversed, and the air flows from the impeller wheel through a downstream opening part 113 and the annular flow channel 111 extending in the upstream direction to be introduced again into the gas inlet 108, so as to recirculate in the compressor.
As a result, performance of the compressor is stabled, and the compressor surge margin and the choke flow are both improved (see “RCC (recirculation compressor)” of FIG. 11).
Further, Patent Document 1 discloses that the inner annular wall 109 and the outer annular wall 107 extend in the upstream direction and house an inlet guide-vane apparatus. The inlet guide-vane apparatus includes a plurality of the guide vanes 114 extending between a center nose cone 115 and the inner annular wall 109.
The guide vanes 114 sweep forward in the rotational direction of the impeller wheel 101 to induce precedent swirls in the air flow which reaches the impeller wheel 101. The precedent swirls improve the surge margin (surge limit) of the compressor. In other words, the precedent swirl flow reduces the flow which causes surging in the compressor. (see the “RCC with guide vanes” of FIG. 11).